Schizophrenia (SZ) is a severe, persistent psychiatric disorder whose molecular underpinnings remain largely unknown. A highly replicated pathological feature identified in postmortem tissue studies is decreased dendritic length and complexity in Layer 3 pyramidal cells (L3 PCs), found across multiple cortical regions. Dendritic length and branching determine a PCs receptive field, help to segment computational compartments, and contribute substantially to how the received signals are integrated and transmitted to the cell body. Thus, there is a compelling need to investigate the molecular pathogenesis of dendritic length and branching alterations in SZ, as these reductions may directly contribute to disease pathology. Dendritic morphogenesis is restricted by signaling via a complex of Nogo receptor (NGR), p75, and the KAL9 isoform of Kalirin (KAL). KAL9 activates RhoA downstream of this pathway. Increased levels of Nogo mRNA as well as elevated levels of KAL9 protein have been described in SZ, suggesting enhanced activity of this pathway may contribute to dendritic impairments in SZ. We have found that a missense mutation near the RhoA domain in KAL9 (KALRN-PT) can model gain of function in this pathway: 1) Enhancing RhoA activity downstream of KAL9, 2) Downregulating expression of genes for microtubule transport proteins and 3) Recapitulating the reductions in dendritic length and complexity observed in SZ. I propose studies to test the hypothesis that enhanced activity in the NGR/p75/KAL9 pathway downregulates the expression of microtubule transport proteins and subsequently impairs dendritic morphogenesis in PCs across development. I have generated a genetic mouse model containing the KALRN- PT missense mutation at the endogenous locus as a novel tool to study enhanced NGR/p75/KAL9 pathway activity. First, I will test the influence of KALRN-PT on dendritic morphogenesis in vivo in L3 PCs across the developmental epoch that coincides with SZ risk in humans. Second, I will use RNA-sequencing on laser capture microdissection samples of L3 PCs from KALRN-PT mice to identify altered transcripts downstream of NGR/p75/KAL9 signaling. Finally, I will test the ability of NGR/p75/KAL9 pathway inhibition to rescue structural impairments in PCs in organotypic slice culture, and subsequently characterize the molecular profile of these rescued neurons to identify specific therapeutic targets responsible for rescue. Integrated with this research project is training to: 1) Develop expertise in mechanisms of impaired dendritic morphogenesis relevant to SZ. 2) Expand knowledge of how pyramidal cell development is influenced by circuits and age. 3) Develop expertise in using animal models of the developmental progression of disease pathology. 4) Expand my skills in advanced analytics and bioinformatics. This unique training will provide me with the necessary background to establish an independent, NIH-funded laboratory making innovative contributions to the molecular mechanisms underlying dendritic impairments in SZ and support an R01 submission during year 3 of my K08 award period.